Why are humans so cruel, mean, and rude?

I know, i'm in my teens and I have been wondering the same thing. Why do we do this to our earth and to others, who would (and why would they want to) kill someone who is trying to help us be better people? Why is there so much hatred about race and differences between us as silly as that? I'm really not sure why we are so careless about the earth like there's another place like it we can simply flee to if we ruin it, or why we choose bad over good mostly. I do know a lot of us do care and try to do the right thing. =/